Emergency break-in system for communication systems



Sept. 12, 1950 E. w. MARLOWE EMERGENCY BREAK-1N SYSTEM FOR COMMUNICATION sYsTEMs Filed March 50, 1948 .N i M EN M QNNNQNY Nm, HQ, m P m MSQQNBMQ @N h d QQWNWWW m T IIII F I L E mi A mw .w T5 T5 h M .wfx H Y l Ww rmi EN .1 n Mm EN :H EMME n WSS n@ NSNPQ .l .M e QM. lm -Il I. BNQ n N@ .w @S m EMME@ j L HNN W WQQMNQNMQ N MS .Nm En @www M E H E nl NL A A. IHI I M .Nkww NQ w E EN EMTMQ mm -u T Patented Sept. 12, 1950 EMERGENCY BREAK-IN SYSTEM FOR COMMUNICATION SYSTEMS Elbert W. Marlowe, Wilkinsburg, Pa., assigner t0 The Union Switch & Signal Company, Swissvale, Pa., av corporation of Pennsylvania Application Much 3o, 194s, serial No. 17,999

(ci 25o- 13) 3 claims. l

My invention relates to communication systems, and particularly to improved means for providing break-in operation of such systems.

When a communication system utilizes a single frequency channel for transmission and reception to which access may be had to any time by a station desiring to transmit, it will be apparent that another station is unable to communicate with the station which is transmitting. That is, communication systems of this type are usually operated on a push-to-talk basis, and during the time a station is transmitting its receiving equipment is rendered inactive, and vice versa. Thus a station which is transmitting cannot hear an incoming message since its receiver is cut oif during the time the transmitting equipment is retained in its active condition.

Such types of single channel communication systems are often used where the receivers of all the stations of the system are retained active during non-communication periods ready to receive a message and a call sent out by any station is picked up by all of the other stations. For example, such communication systems are used on aircraft, for railway train communications, etc. It is quite often necessary for a station to send out an emergency message that should be received without delay by all of the other stations, at least those stations within the usual range of transmission. With the pushto-talk arrangement heretofore used, a calling station has no way to break-in on a station whose transmitter is switched to an active condition and this station would not receive an emergency message.

Accordingly, an object of my invention is to provide a single frequency channel communication system having voice operated switching means selectively responsive to audio frequencies modulated at syllabic frequencies, and having call signal means for generating audio frequencies modulated at syllabic frequencies to actuate the voice operated switching means.

A further object of my invention is to provide a communication system incorporating a novel voice operated switching means for switching the transmitter on, and which switching means is responsive only to the syllabic frequencies present in speech, and not responsive to noise,

2 break-in system which does not interfere with the normal use of the equipment.

A further object of my invention is to provide a break-in system for communication systems of the class described, which will be rapid in operation.

A further object of my invention is to provide a break-in system in which the transmitter of each individual station is switched on only when the transmitter is modulated, so that the receiver is rendered active during short pauses in the speech of the person operating the equipment.

Other objects of my invention and features of novelty therein will be apparent from the following description taken in connection with the accompanying drawings.

In practicing my invention, I provide at each station in a communication system which employs a single frequency channel, means for switching on the transmitter and switching ofi the receiver when and only when the operator is speaking into the microphone. During pauses in the speech, the transmitter is switched oi and the receiver is switched on. In order that the equipment will not be responsive to noises other than speech, the switching apparatus is made responsive only to sounds modulated at syllabic frequencies.

I shall describe two forms of communication system embodying my invention and shall then point out the novel features thereof in claims.

In the drawings, Fig. 1 shows a diagrammatic view of a station in a single frequency channel communication system embodying my invention, and Fig. 2 is a fragmentary view of a modification of the apparatus shown in Fig. l.

Similar reference characters refer t0 similar parts in each of the two views.

It is to be understood that although this system is shown and described in connection with a space radio communication system, it is readily adaptable to carrier current signaling or inductive communication Systems.

Referring to Fig. l, the apparatus shown may be divided into three portions, a transmitting portion, a receiving portion, and a switching portion. High voltage direct current energy is supplied from a source not shown, the positive terminal of which is designated by the reference character B(+) and the negative terminal of which is grounded. Grid bias voltage is also supplied from a source not shown, the positive terminal of which is grounded, and the negative terminal of which is designated by the reference 5 3 character C(-). To simplify the drawing, the circuits for supplying energy to the heaters of the various tubes have not been shown.

The transmitting portion or transmitter comprises a microphone MIC, a voice frequency band pass filter VF, a call signal oscillator CSO governedby a push button PBLa modulatorusername; Mo; keygen. inducing' me :vacuum tube VTVy a power 'aiplifier "PA and a' transmitting antenna TA. The transmitter may be arranged in any well-l-2nown manner, and may' be of the frequency modulationitypa.theampliaccording as the system employs frequency modulation, ampiitude modulation, vor some ncient to point out that the voice frequency energy from the microphone MIC ismpasse'd through the voice frequency band passfilterVF' to the modulator-cscillator MO Where a center or carrier frequency gemerated therein is modulated by the ybicefrequency:energyf Them'odulated cari-ier frequencyfis suppliedtlirouglil the keyertube V TI- toA thepnwer'amplier'ln Where itl is'a-mplied and-supplied"tothe transmitting antenna TA; i

The keyer tube VTI is a multi-gridvacuum tubewhich is arranged rand constructed sotthat the modulated carrierifr'equencyenergy which is'supplied'to a-nrstcontrol'f'grid 5 'causes corresponding variationsin"'thecircuit" including the plate 'i of` tube VTIwhen andpnlywlien a second controlvl gridiSlhas-*a 'positive' potentiai with respect to the cathode II. Accordinglyjit will oe-seen thatvtheinodulated carrier`energy is'supplied tothe --power amplifier "PA only when the control gridil of A"tube-VTi is d-riven'positive with respect to the cathodeII; sothat"energy is radiated -fromthe-antennaTA'only at such time as the grid A9 ofvacuumtube VTI is positive with respect'toits'cathode rI I. The call signal-oscillatorCSO is arranged and constructed so that when-contact i3' of push button PBI is operated, high -voltageenergy'is supplied -to Athe oscillator"to therebygenerate a high-pitched" audio 'frequency wvhich" is"sup plied`-tof` the modulator-oscili'atoi" MO. At the same ltirne, contactNiE' ofL pushbuttoriPBI 'coni nects'the grid 9- ofvacuum' tubeiVT't to: a'low voltage positive potential whichis'pr'oducedby theA voltage divider V'comprisingd'esistorsti 'and RZ lconnectedy acros'sthepowersource. "At this time, -therefore, theearrier frequency generatedby the-modulatoreoscillator*MO:is' modulated oy audio vfrequencyJ *energy snpitrlied'y thereto "from the @au signal osciiifior pso; whii'e'ni-egrid '9' of vacuum tube VTIl'is driven positive, sothat' the modulated carrier -frequencyf energyvl is'supplied to the powerfamph'erPA and is thereafter radiated fron'iy e antenna TA.

The receiving portdn" or' receiver 'ofthe equl`pment compri-ses a receiving antenna RA, a demodulatoramplifiei""DA, a'n'"au'dio amplier including vacuum tube VTZ, an output ainpliiier OA, and a loud speake'fL-S. "Ih'e" demo'dula'toramplifier DA maybe'of any of the' types wellknown in the art, suchfas; for'exarnple, a heterodyne receiver or a"'tu`n ed radio frequency're' ceiver; and may be of'the freduencyrnodulatiom amplitude modulationor 'other type' to match the type of transmitter modulation which is used, The actual constructionand operation of the demodulator-an'i'plier" DA' 4is' not necessary tov an understandingof myin'vention; and it is deemed sunicient to point-out' that an v`inccimirig 4 modulated signal wave is picked up by the receiving antenna RA and supplied to the dernodulater-amplifier DA where it is amplified and deinodulated. The audio frequency output of the 5 device DA is supplied to a nrst control grid 25 of the audio amplifier tube VT2. This tube is of the multi-grid type and is similar vin constructiomand arrangement to `,that :previously "described in connection with the keyer tube VTI, so that the audio frequency energy supplied to the first control grid causes corresponding *..i-variations i-n the-circuit including the plate 2l tude modulation type, or other suitable type,

ifwhen'andonly when a second control grid 29 has a positive potential with respect to the ,iscapacity coupled to the output amplifier OA, the output of which is supplied to the loud -speaker LS.

From the foregoing it will be apparent that go audio frequency energy from the demodulator amplifier DA is supplied to the output amplilier OA aridin turn ttheloud'SpealierLSti/heniid only when the 'se'cond'control grid 29' is"^driv`en positive with vrespectito' the cathode' 3| of' tribe VT2. i'

The switchingportion o'fth'e Aapparatusincludes anisolating amplierVTh'aving 'a""pri mary windingi of a voice' freq'uency"`trans foriner'VFT in' its pl'at'e'circuitl a' vfull iv'ave'rectifierl RX,"the inputterr'ninals of'w'hich aie'conn'ected' tothe" secondary Winding"3l of transformer VFT, -a syllabic"frequency filter SF sup'- plied vwith"ei'iergy "frorii'the outputh of'theiill wave' rectinerRX; adete'ctor tube V'l`4jan`d 4ta trigger 'circuit including thevacuum tubes" VTS and VT$,"vvhich"ar'e Aheresho'vvn as'being con# tained within a singleenvelope.

'The purpose of theswitching' means lsto provide for switching the traiismittei"'or`i"nd 40 switching the'receiver off during such timesthat the' operator is speaking into'the' microphone MIC, the receiver"being'normally switched on. To accomplishthis` objective, th'e 'apparatus .iS designed and 'arranged so'that' only those frequencies which are peculiar to speechfkno'wri as syllabic frequencies, are able to cause" the switching means to operate. "Syllabidfrequency as `"here Yconsidered is'the frequency of'the envelope of a soundl Wave 'p'roduc'ed'by speech, fn and it'has a'range of 1A2'tof25 cycles per' second. ln otherwords, syllabic'frequency is 'an envelope frequency created on a`v'oice"frequency current due to the syllables of speech.

The isolating ampliflerVT3` is an amplifier connected in'ithe usual manner and serves'to isolate the switching 'apparatus from theremainder of the equipment. 'The' input of'the amplifier tub'e`VT3 is'connected acrossth'e'outputoi the 'voic'e'filter'iVFjin multiple' vwith 'the (m input of' the modulato-os'cilltor'MO and lthus it 'receives a'portion of the voice' frequency energy. The o`utput` circuit of theisolating arnpli'er includes 'the primary 'Winding "35 of the voice frequency` transformerVFT, andv it will be apparenf'that such'an arrangement 'provides alternating currentl coupling'so that only alternating voltages 'appear 'acrosslthe secondary winding 3'1 of transformer Rectifier RX recties the voice frequencyenergyn thus deriving the envelope orsylla'oic requencies," By using aiull Wave rectifier, `the"frequencesof the fundamental components or' carrier 'components o f 'the Speech areA doubled, thereby `en abling the"iise of a 's'imple" filter ',i'ollovving the rectifier to eliminate lthese".frequen'cies.This

WMM!) llumrwmpnent and suv i ,y uw aboveithisfralue., The outputs! this titer isupled tothe Dietetici the ydetector :by a tenacity sounlinsineiworhiars meti Vif the compensator the @optent Synthie, irenuencanlter :BF imtw'lppued Hf Qfilftuboe; This coupling the,;loediresist0r 38,mhieh provides a load rombo inter` sr, the `condenser C)` yand resistorl R4.4 Ac-y mlyfomy, alternating current energy at kItalia*frequencies `is supplied to thelplate 4I oi'd'thel'detector tube VT4, vvliichisl an ordinary and `operates :to rectifyv the alternating current suppliedthereto. It apparent toothose skilled inthe art thatother types of asymmetric units lmay `be employed asthe deior example. a, crystal type of rechner- `Therectified energy presentfat theilcathode` 4170i tube yVT4 issupplied tov kan integrating l network the output or which issuppned to the' l grid 45 of vacuumtube VTltwhich is part of trigger circuit to be subsequentlywdescribed.

Thigcouplim rtwork comprising Athe resistors R6 andthe condensers 'C4' andy C5, is

arranged` so that the potential of resistor .R large so that the discharging time constant for coondenser C4 vis relatively long. a result, the value of ,energyl supplied to the kgrid 45 of tube VTS remains substantiallyV constant duringva talk-spurt!! u The particulary trigger circuitshown is of the Schmitttype, which is shownand described :in

Time Bases, by O. S. Puckle, pubin the United States by John Wiley 8i Sons, VThis circuit is Vcharacterized by .having twoA states, and will assume oneorithe other of ffffs'tates depending upon the instantaneous pqtfentlal on the 'grid of one of its tubes, in this the'grld 45 of vacuum tube VTS. In the norst'ate. that is 'with the absence of a positive potentialjon the grid 4,5;,th'e parts are'arrang'ed 6j: t0 ailrstthrehold potential-which i'ahcve thenotentisl; 01: thecathodeotube -llfrlstart to and Aas a result; its Aplate.potential M118` Thlssfall-in .potential resultsfinsa :negative voltage; being Supplieditocthefgrid 41e oi tube due to thebias voltage `source over, the-positiveopotential supplied thereto through resistors R1 and/11.8,. .with a vresult that tube VTS; is cut oil. The `circuit will remain in stablel state ,until thervalue `of the,l potentialion the grid 45 of 1 vacuum p tube VTS falls-'below a second critical orthreshold value, which may range from a `value lalmost as highs theilrst :threshold potential to a value considerablyv lower. Vand `vvhich'is determined `by the adjustable resistor RID. The digilerence between the :liirstz and second threshold potentialis calledufback-lash and is characteristic `of theschmittftrigger; circuiti: This characteristicis employed mima-system' to increasetheeiective discharge time constant of condenser C5 and thus increaseithe `disparity .between the charging ,and` discharging time4 constants .-more thanwould be possible if the trigger circuit: were of thev type 'fwhich' switched from-one state to the other 'state at nearly the same potential of grid 45.f-'As a re sult, the detector `output integrating network comprising thehcondensers "CL and-C5 andresistors R5 and R6 is simpliiiedf and properlop eration thereof is betterinsuredl f The second controlgridl of audio ampliiicr VT! is connected in parallel with gridf 41 vof` vacuumA tube that both grids have the same potential impressed thereon. It wlllbe apparent thereforepthat since the grid 41 of tube VTS normally has a positive potential as explained above, -thelgrid 29 ot vacuum tube VT! will also have a positive potential. and, as previously explainedpwill permit tube VTZ to con"- duct. Accordingly, inthe normal condition; an incoming-signal `picked upy by` antenna RA and' supplied to the demodulator-anrplifler DA will be demodulated and ampliiied and a, resulting audio frequency energy will'be passed by tube VTZ to the output amplier OA and thence to the loud speaker LS to audibly reproduce the incoming signal. Y

A voltage divider comprising lresistors RI I, RI 2, and RIB betweerrthe positive terminal B(+) of` the high voltage power supply and the negativeterminal C() of the grid 4bias voltage supply isconnected with-the plate of tube VTS. 'I'he second control grid 9 of keyer tube VTI is connectedto the junction 5I ofresistors RI2 and' RIS.` The parts are proportioned and arranged so that with the` trigger circuit inl its normal state lwith vacuum tube `VTS "conducting, `the' negative potential of the bias voltage supply is greater thanthe positive voltage` from thelhigh voltage power supply at the junction Sli and a negative bias voltage is applied tothe grid 9 of` the keyer tube VTI; Accordingly, when the' trigger circuit is in its normal state, the keyer tube VTI iscut on, so thatthe transmitter is ren- A deredinactive. y

From the foregoing description, it will be seen" that ther transmitter is normally lnactive,and the receiver is; active,` so that incoming messages may It'will now, be assumed that the operator de-l sires to transmit a message,y andfaccordingly' speaks into themicrophone MIC".y 'I'he voice fre; quencies y of ythe audio f frequency energy supplied from" the microphone i MIC 'are' 'Iiilteijed by,I 'the voice: frequency .fijlidipass VEditerariel-are then supplie'dito the modulatbr'scillator MO; to there byfmodulatef the ic'arr-ler -eqiencylandthemrulated. carrier'lfreejue 1.1"eyl isf supplied"7 to'tlie' gr-id'-r -5 o'i" the. keye'rltiibeVT-I Additionally; theffvoice frequencyenergy: -issupplied'to the isolating1l am@ pll'erf .EVT-3'; and the@ amplified-i voiceffreduenc'y energ`y'iis 'thereafter .--re'ctifiedf by tliefull? wave rectier .RX landi supplied to the svll'ab'ic fre-l quency-ifilter SE-Swh'ere' the' fundamental-*frequency componentsofthe sp'eechiareiiltered out, and! the'f speech. envelopes or. syll'abc ffre'quen'cies' are :fpass'efd to :the: detector: tube/VT4; where they areerectied .fand then-Supplied through the lin'- te'gr'ating network .tof the cgrid A51 ofi vacuum'itube VT5. 'W The; voltage supplied;v tot the 'gi-id 45'. ofs'vafcuuzn tub'ef-VTS at thisftirne'. is; su'iicient toi caiuselfthe triggerfcircuit to switch' tof. its second'.` statief:3 As previously explained.the Apolarity-of` the-bias volt#- ages ofifgrid .9 ofi vacuum tube VTI f 'an'dzfgrid .7Bf ofxvacuumfftube. :VT2 change when"v thev trigger circuitvoperates from its normal state"to'litsls'ee;` ndstate; sov that .vacuumA tube fV'Il' is rendered conductive and vacuum tube'VT2l is cut-oif.

1: Asaf result; at this; timev the receiveris rendered inactive, sof that incoming signals are" not reproduced fin the loud speaker LS, andthe transmitter is rendered?active,.so that ythe modulated carrierf frequencyY supplied fromfthe modulator. oscillator MO through'ikeyer VTI; to; the ...poweri a-mpliiier PA. is transmitted? thereafter-from'fthe antennavTA. 4 i

f As previously explained, thecoupling network associated with detector tubevV-Tii, andthe trier: ger circuit are arranged to bridge. gaps'between syl1ables-of speech, so that the apparatus lremainsin its'second 'state asfdescribed above until apause occurs in;the.conversation,` or in other words until the end ofa-talk-'spurt.

Atthe end of a talk-spurt, syllable frequency. energyiis nolongersupplied to th'eadetector VT4., andv as af :resultthe voltageof, the; energyl supe plied to. thegrid 45 of-'tubeVT5rfalls.to zero, andthe trigger circuit reverts to its normal state.H

Whenthe trigger circuit returns to itsfjnorma'lstatethe vacuumtubefV'Ill` will be cut offA and vacuum tube VT2 will be rendered conductive as previously;v explained; so that 1the=transmitter is; again= rendered inactive, f and: the: receiver is. rendered active.

It. will be apparent therefore that duringitlie; time the message is being ytransmitted 'from' the station,Y the receiver is rendered inactiveonly atY such times asythe operator-:is speaking into-l the microphone, and thatrduring` ,pauses in his conversation orl speech. the transmitter is tre'or-,t dered inactive and-the receiver. is renderedacr tive, and any'fincorning signals"willbe:heard byl ther operator during/fthe pauses in-'his'.fsp'eech.v

If during. the time thatytlfie.V operatoris trans.n mitting. a message; a .second` station-:desires to; break-.in .onhis transmission, theoperator.- at the; second station depressesgthe push buttongPBl-l associatedvvithihis equipment, so thatacall sig,- nal is transmitted. At the transmitting. station,v during the pauses Y .of speech of 1 the t,ranusinitting operatorfhe can hear the breakin.or aait-sig; nal'th'rough the 'loud speaker.I Thevbreak-finsige nalis of'cursere'ceive'd inthe usual'niannerat all. stations which are receiving, .but as -previou'sly.l explained 'the break-in" s ysterlil as herein shown' and described :is advantageous. in thattle" operatof'at'the l transmittingstatiori, illbe*'noti"-l fied"that.anttneiv'v tion'dsires to'fbr'e'ak mstmnsimssion n n r 'teer'anngneiwork @essaierai warf damier.l VT4 `t0-fa. surment-vaio i0@ f naevacuummbe vTsytf-rbejiripped in the artand as:here:shown=isfaHartlvityii lout thativhen energy 'is supplied tothe platei'gand' apositive-potential'is"suppliedf-tc' 'the grid' Bl f tube VTi, tha-oscillatorvfgenerate'senergy audio "fiequencyf range;`l say; l"for" example, Q00L cycles perisecond, and this ei'iergy'is s'i'ipjiPl-ied"I to the-Secondary rWinding? Eflf transformer 1, *The gas tube VIlis proportionedand arranged tofunc'tion` asl a-frelaxation-1oscillator `r hai/irre a fred-uency within the-syllabicffreqency range," servi-foi*example,` l0 cycles-*per-second.

Incineration, if the operator" desiresto'v transl' mit call signal/he Yd'epfifes'ses the )Sinsl'buttonVV PBI. Energy is'rth'ereby supplied Ato the"relaxa-l tion oscillator and theaudiooscillaitorj."Tlieconi denser Cl" is i chargedthrougli' the` charging re, sistorRifiV until-'the voltage across thecondens'e'ri exceeds tli'e ionizing voltageof tube VTT, and when the Avoltage exceedsJ the ionizing. voltage: or` time vfrigmec-tube 'connects anni? the p01*? tential is reduced1 bei-civY its deioniaizigj"v potential'f at which time it beeomesextinguishedf""rnecyci' is"rereated as long" as power Vis supplied to I tlfi'e circuityand as' previously I stated, 'the 'parts' are' arranged and proportioned' soI thattli tiili`I aboutten timesiper second. 'rneridefjeftube VTS' isjconnected.tocondehser"CI' 'so v'tlfia't thefj voltagein'ipre'ssedfo-n giid'flilvaris iii acc'oraiicey with.` .the ."cond "ensefrlI voltage; I'Resis'tor` El 'llfs'erves 4, tli'e.. dual function o'f' alg' 1"d roia'sfresistor for' gridl si andar.. ai biederjfdr condenser ci. "f1twiii beiseen therefore th-arjan onsI time' trie gestoten-j marbrerie si Qf :tube vvra is varied about; 1o

times per. second by thebperation of' ther'elaxal' tion oscillator, so 'that'.the 'audio oseillatoris rale ternately 'rendered' operative 'and inoperative second. Since this energy has the same frequency characteristics as speech, namely, in the voice frequency range and modulated by a frequency in the syllabic frequency range, it will be apparent that the energy supplied from secondary winding 51 will cause the switching means to render the transmitter active and the receiver inactive. As a result, a cali signal is transmitted which when received and demodulated provides a distinctive calling signal.

Although I have herein shown and described only two forms of communication systems embodying my invention, it is to be understood that Various changes and modifications may be made therein within the scope of the appended claims without departing from the spirit and scope of my invention,

Having thus described my invention, whatl claim is:

1. In a communication system, in combination, a transmitter operative at a given carrier frequency and adapted to be modulated by voice frequencies, a receiver operating at said given frequency, switching means eiective when rendered operative for selectively activating said transmitter and deactivating said receiver, said switching means comprising means responsive to frequency variations in the syllabic frequency range of the voice frequencies supplied for the modulation of said transmitter and having a timing characteristic such that the switching means becomes operative upon the supply thereto of audio frequency energy having a component in the syllabic frequency range only and remains operative during relatively short interruptions in the supply of said energy, said timing characteristic being such that said switching means does not operate upon the supply thereto of random bursts of energy in said syllabic frequency range, and call signal means comprising means for at times supplying said transmitter and said switching means with energy having a frequency within the voice frequency range and modulated at a frequency within the syllable frequency range.

2. In a communication system each station of which includes a transmitter and a receiver operating on the same carrier frequency, the combination comprising; switching means including a rectifier, a high frequency cut off filter, a detector and a trigger circuit connected in the order named; said switching means effective to create bias voltages in the output of said trigger circuit in response to the syllabic frequency of a voice frequency current applied to said rectier, circuit means to connect the input of said rectier to the input of said transmitter to receive a portion of a voice frequency current supplied to said transmitter, other circuit means to connect said trigger circuit to said transmitter and to said receiver to apply said bias voltages thereto, and said other circuit means disposed for said bias voltages to render the transmitter active and the receiver inactive or vice versa according as the voice frequency current is or is not applied to said rectifier.

3. In a communication system each station of which includes a transmitter and a receiver operating on the same carrier frequency, the combination comprising; switching means including a rectifier, a high frequency cut off filter, a detector and a trigger circuit connected in the order named; said switching means effective to create bias voltages in the output of said trigger circuit in response to the syllabic frequency of a voice frequency current applied to said rectifier, circuit means to connect the input of said rectifier to the input of said transmitter to receive a portion of a voice frequency current supplied to said transmitter, other circuit means to connect said trigger circuit to said transmitter and to said receiver to apply said bias voltages thereto, said other circuit means disposed for said bias voltages to render the transmitter active and the receiver inactive or vice versa according as the voice frequency current is or is not applied to said rectifier; call signal generating means comprising an oscillator capable of generating alternating current having a frequency within the voice frequency range; modulating means for modulating said oscillator at a frequency lying within the syllabic frequency range, coupling means for coupling the output of said oscillator to the input of said transmitter and to said switching means; and manually controllable means for at times energizing said oscillator and said modulating means.

' ELBERT W. MARLOWE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,298,613 White Mar. 25, 1919 1,916,016 Rives June 27, 1933 1,970,423 Frink Aug. 14, 1934 1,975,270 Hansell Oct. 2, 1934 2,101,668 Bishop et al. Dec. 7, 1937 2,144,936 Rochow Jan. 24, 1939 2,213,398 Kircher Sept. 3, 1940 

